BLOOD VESSELS OF LUMBRICUS. 77 



therefore, seem to confirm the results of physiological experi- 

 mentation. No effort was made to trace the development of 

 hearts in the regenerated heads and the final reorganization of 

 the circulation, and it is doubtful whether the worms would have 

 lived long enough for this purpose. 



It is probable that the failure of the blood vessels to adjust 

 themselves to the new conditions is at least one of the c chief 

 causes of the death of worms under these experiments. The 

 continued strong pulsations of the dorsal vessel after the 

 removal of the hearts force the blood out through vessels which 

 normally empty into it. In some cases the reversal of flow 

 through the vessels of the body wall and intestines is produced 

 readily enough to allow the worm to survive the operation, but in 

 most cases less blood would reach the ventral vessel than is 

 necessary to supply the posterior end of the worm and an insuf- 

 ficient amount of blood would pass through the respiratory plexus 

 beneath the hypodermis. The blood which leaves the dorsal 

 vessel in the anterior part of the worm either settles in the vas- 

 cular layer of the intestine, which readily expands to receive it, 

 or passes directly through the parietals to the subneural, which 

 is consequently greatly expanded ; and avoids the respiratory 

 plexus because of the resistance in that quarter. A similar with- 

 drawal of blood from the respiratory plexus of the posterior end 

 of the worm also results indirectly from the small amount of 

 blood in that region, so that the whole worm is seriously deprived 

 of needed oxygen. In the normal circulation the blood is driven 

 to the respiratory plexus from the ventral under direct pressure 

 from the hearts, and there is no other way of less resistance by 

 which the blood may return from the ventral to the dorsal. 

 Upon the view of the circulation held by Bourne ' and Harring- 

 ton, 2 namely, that the dorso-intestinals empty into the dorsal ves- 

 sel and the parietals carry blood away from it, it is evident that 

 the path of least resistance from the pulsating dorsal vessel is 

 through the parietals directly to the subneural and that there 

 would be nothing to drive the blood through the respiratory 



1 Bourne, A. C., "On Megascolex cceruleus and a Theory of the Course of the 

 Blood in Earthworms," Q. J. M. S., Vol. 32, p. 49, 1891. 



2 Harrington, N. R. , " The Calciferous Glands of the Earthworm, with an Appen- 

 dix on the Circulation," Jour. Morph., Vol. 15, Suppl., p. 105, 1899. 



